class dd_slider_meta {

	public function __construct() {

		if ( is_admin() ) {
			add_action( 'load-post.php',     array( $this, 'init_metabox' ) );
			add_action( 'load-post-new.php', array( $this, 'init_metabox' ) );
		}

	}

	public function init_metabox() {

		add_action( 'add_meta_boxes',        array( $this, 'add_metabox' )         );
		add_action( 'save_post',             array( $this, 'save_metabox' ), 10, 2 );

	}

	public function add_metabox() {

		add_meta_box(
			'slider',
			__( 'Slider Options', 'text_domain' ),
			array( $this, 'render_metabox' ),
			'post',
			'normal',
			'default'
		);

	}

	public function render_metabox( $post ) {

		// Retrieve an existing value from the database.
		$slider_caption = get_post_meta( $post->ID, 'slider_caption', true );
		$slider_link = get_post_meta( $post->ID, 'slider_link', true );

		// Set default values.
		if( empty( $slider_caption ) ) $slider_caption = '';
		if( empty( $slider_link ) ) $slider_link = '';

		// Form fields.
		echo '<table class="form-table">';

		echo '	<tr>';
		echo '		<th><label for="slider_caption" class="slider_caption_label">' . __( 'Caption', 'text_domain' ) . '</label></th>';
		echo '		<td>';
		wp_editor( $slider_caption, 'slider_caption', array( 'media_buttons' => true ) );
		echo '			<p class="description">' . __( 'Input your caption for slide (HTML tags are allowed).', 'text_domain' ) . '</p>';
		echo '		</td>';
		echo '	</tr>';

		echo '	<tr>';
		echo '		<th><label for="slider_link" class="slider_link_label">' . __( 'URL', 'text_domain' ) . '</label></th>';
		echo '		<td>';
		echo '			<input type="text" id="slider_link" name="slider_link" class="slider_link_field" placeholder="' . esc_attr__( '', 'text_domain' ) . '" value="' . esc_attr( $slider_link ) . '">';
		echo '			<p class="description">' . __( 'Input the slide URL (can be external link)', 'text_domain' ) . '</p>';
		echo '		</td>';
		echo '	</tr>';

		echo '</table>';

	}

	public function save_metabox( $post_id, $post ) {

		// Sanitize user input.
		$slider_new_caption = isset( $_POST[ 'slider_caption' ] ) ? wp_kses_post( $_POST[ 'slider_caption' ] ) : '';
		$slider_new_link = isset( $_POST[ 'slider_link' ] ) ? sanitize_text_field( $_POST[ 'slider_link' ] ) : '';

		// Update the meta field in the database.
		update_post_meta( $post_id, 'slider_caption', $slider_new_caption );
		update_post_meta( $post_id, 'slider_link', $slider_new_link );

	}

}

new dd_slider_meta;